1. Field of the Invention
This invention relates generally to prosthetic devices for use in the reconstruction and/or repair of the human vascular system. More particularly, the invention relates to vascular grafts derived from biological tissues, and methods of making the same.
2. Description of the Related Art
Vascular grafts have become widely used for replacing diseased arteries in humans. For example, a blocked portion of an artery can be surgically excised and a tubular shaped vascular graft implanted in its place. Alternatively, a vascular graft may be used to bypass a diseased vessel entirely. Among the most common vascular graft prostheses are those formed from synthetic biologically compatible materials in a tubular form which retain an open lumen to permit blood to flow through the graft after implantation. The biologically compatible materials typically used to produce synthetic vascular grafts have included thermoplastic materials such as polyesters, polytetrafluoroethylene (PTFE), silicon and polyurethanes.
Unfortunately, certain limitations have been associated with the use of synthetic vascular grafts. For example, synthetic grafts are often highly susceptible to neointimal hyperplasia and late graft thrombosis. Moreover, many synthetic grafts have a relatively high porosity and must be pre-clotted prior to implantation to avoid extensive hemorrhage. This pre-clotting procedure is not always practical or successful.
In addition, many synthetic vascular grafts have a tendency to kink or collapse when the graft is twisted or bent during or subsequent to implantation. To address this problem, many synthetic grafts are subjected to a crimping process. Crimping involves forming ridges in the walls of the grafts to minimize the likelihood of kinking or collapse of the tubing when flexed. Most crimping processes result in uniform, regular, circular corrugations along the length of the graft which help maintain a more uniform strength over the surface of the graft tubing. For example, U.S. Pat. No. 3,878,565 describes a tubular textile synthetic fiber prostheses wherein the graft body is crimped into regular, circumferential corrugations. U.S. Pat. Nos. 3,945,052, 4,047,252, and 4,517,687 describe knit and woven grafts that are circularly crimped. U.S. Pat. No. 4,892,539 describes a graft that is crimped in a spiral fashion.
To overcome some of the limitations associated with synthetic vascular grafts, grafts derive from biological tissues have also been under investigation. These grafts are typically derived from animal tissues which have been treated by a suitable process such as a chemical or photofixation procedure in order to stabilize the tissue and to improve its biocompatibility. However, tissue-derived arteries can be susceptible to degradation in vivo resulting in aneurysm and device failure. To provide protection in the event of aneurysm, the tissue graft may be enclosed within a tubular external sleeve. This sleeve may be slightly oversized with respect to the outer diameter of the graft and made from a non-compliant, expanding material that is resistant to degradation upon implantation. In the event of a growing aneurysm, the sleeve serves to contain the artery and prevent bursting of the graft.
However, we have found that the use of tissue-derived grafts in combination with synthetic external sleeves, presents a unique obstacle. Although it is desired to provide a tissue artery which retains the longitudinal compliance of a natural tissue, upon inserting the compliant tissue graft within a substantially non-compliant synthetic sleeve, there is a mismatch of the sleeve to tissue length once the graft is pressurized. Moreover, our attempts to match the compliance of the sleeve with that of the tissue graft using conventional crimping procedures resulted in vascular grafts having incomplete healing responses following implantation.
The present invention is directed to providing tissue-derived vascular grafts which overcome, or at least reduce the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a vascular graft is provided which comprises a length of vascular tissue having a tubular structure and a synthetic sleeve enclosing the vascular tissue. The synthetic sleeve has an extended length and a relaxed length, in which the relaxed length is less than the extended length and the relaxed length is substantially similar to the length of vascular tissue. The relaxed length of the synthetic sleeve is typically between about 50% and 95% of the extended length of the synthetic sleeve, more typically between about 70% and 90% of the extended length of the synthetic sleeve, and preferably between about 75% and 85% of the extended length of the synthetic sleeve.
It has been found that it can be beneficial to provide crimps along at least some portion of the longitudinal length of the synthetic sleeve. The crimps are preferably circumferential crimps that are substantially rounded, i.e., do not have any angular features which give rise to void spaces between the sleeve and the graft tissue following implantation and pressurization of the device.
The graft tissue of the vascular graft may be derived from any of a variety of biological sources, e.g. human, bovine, porcine, ovine, equine, canine or goat. Vascular tissue derived from a carotid artery or thoracic artery, for example from a bovine, are particularly suitable. Prior to use in a vascular graft of this invention, the tissue is treated by one or more fixation techniques to stabilize the tissue. The fixation of the tissue may be performed by any of a variety of conventional approaches, such as chemical fixation using glutaraldehyde, or, preferably, by a photooxidation or other process which allows for some retention of the mechanical compliance of the native tissue before fixation.
The synthetic sleeve may be comprised of a wide range of polymeric materials that can be woven, knitted or braided polymeric fibers or yarns in the form of a tubular structure. These may include polyesters, polypropylenes, polyethylenes, polyurethanes, and other like materials. Preferably, the synthetic sleeve is comprised of polyethylene terepthalate or polytetrafluorethylene.
In a further aspect of the invention, there is provided a method for making a vascular graft, in which a tubular polymeric sleeve, e.g., a polyethylene terepthalate or polytetrafluoroethylene sleeve, is longitudinally compressed to a length less than about 50% of its initial length and exposed to a first heat treatment to introduce crimps in the sleeve. Thereafter, the sleeve is stretched longitudinally to a length between about 70% and 90% of its initial length and exposed to a second heat treatment to lessen the crimps and thereby provide substantially rounded crimps. A length of vascular tissue, preferably photooxidized vascular tissue, is thereafter inserted within the sleeve to provide a vascular graft device.